A conventional stone separation table (SST) 10 manufactured by Franz Grimme Landmaschinenfabrik GmbH & Co. KG, Wiesenstrasse, Postfach 1280, D-2845 Damme/Dummer, Germany, and sold under the trademark "ROLLER STAR" (TM), is illustrated in FIG. 1. The SST incorporates multiple sets of rollers 12 in adjacent sequence for transporting root crops from one side of the table 10 to the other side. Each set of rollers 12 includes a first conveying and cleaning roller 14 with projecting fingers 15 intermeshing with a second separating roller 16 also formed with projecting fingers 18. The projecting fingers 15 of cleaning rollers 14 clean dirt and mud from the projecting fingers 18 of separating rollers 16. The cleaning rollers 14 may be constructed with fewer projecting fingers 15, e.g. a six finger star roller, than the separating roller 16, e.g. a 12 finger star roller. The rollers 14 and 16 are driven in the same direction of rotation for conveying potatoes or other root crops across the tops of the projecting fingers. The projecting fingers are typically curved in the direction away from the direction of rotation of the star rollers.
The third roller of each set of rollers 12 is a counter rotating pinch roller 20 having a substantially smaller diameter than the separating roller 16. The smaller diameter counter rotating pinch roller 20 causes a level drop off following the separating roller 16. The larger root crops are passed by the tips of the projecting fingers 18 over the smaller pinch roller 20 to the next set of rollers 12. The spacing of the projecting fingers 18 of separating roller 16 receives smaller stones, clods and dirt between the fingers for delivery to the "pinch" valley between the counter rotating rollers 16,20. The stones, clods and dirt are "pinched", rotate downward, and are separated by dropping below the SST.
Because of the counter rotating pinch roller 20 of significantly unequal and smaller diameter, it is necessary that the second set of rollers 12 in the downstream conveying direction be positioned below the first set 12 for continuous conveyance of the root crops. As a result the SST conveyor must be constructed with a diagonal or downward pitch to be operational. An SST might typically be composed of two or three sets of rollers oriented in a downward stepping or diagonally downward direction.
A disadvantage of the necessary pitch associated with the prior art SST 10 is that it cannot be incorporated in the United States style potato harvester formed with horizontal planar conveying levels. Such potato harvesters are manufactured for example by lockwood Corporation of Gerring, Nebraska; Logan Corporation of Logan, Iowa; Art's Way of Armstrong, Iowa; Double L Manufacturing Company, Inc., American Falls, Idaho; and Thomas Equipment of Centerville, New Brunswick, Canada. Another disadvantage of the Grimme type SST is that the counter rotating pinch roller 20, despite its small diameter, may still pinch and bruise root crops passing over the downwardly pitched conveyer.